Current and past methods of atmospheric metal sulfide leaching can be hindered by the formation or build-up of froth over time within a leach circuit. The froth may, for instance, build up near top portions of a leach reactor, and accordingly, portions of a metal sulfide concentrate which are to be leached, may be displaced from and therefore may leave contact with lixiviant. Accordingly, some ground sulfide particles within a metal sulfide concentrate may not be exposed to lixiviant for the predetermined residence time necessary for complete leaching to occur. It is not uncommon for froth contents, which are displaced from lixiviant, to contain un-leached or partially un-leached particles. Such effects reduce overall leach recovery, and/or may decrease actual leach residence times for floated particles contained within the froth, to below that which is required to achieve complete metal dissolution (e.g., for complete copper dissolution). In short, conventional atmospheric metal sulfide leaching may be impeded by froth formation. Without limiting the scope of this disclosure, it will be understood by those skilled in the art, that the term “atmospheric” or “substantially atmospheric” where used herein may include systems or apparatus within a leach circuit which may negligibly contribute to the overall use of above ambient pressures. For example, without limitation, open-top stirred reactors and pressurizable enclosed stirred-reactors may be present within an atmospheric or substantially atmospheric leach circuit according to some embodiments, without limitation. Without departing from the intent of the invention, the reactor head space may be atmospheric or alternatively pressurized to above ambient pressure to control the head-space gas composition. The pressure may be controlled by temperature or by an applied gas pressure that is above ambient pressure. As will be described hereinafter, in some preferred embodiments, most leaching may occur at atmospheric pressure conditions, and a much smaller amount of leaching may occur at above atmospheric conditions. In some preferred embodiments, a majority of leaching residence time of a metal sulphide particle may occur at atmospheric pressure conditions, and a minimal amount of leaching residence time of a metal sulphide particle may occur above atmospheric conditions. For example, in some non-limiting embodiments, a leach reactor 202, such as the one shown in FIG. 2, may comprise one or more open-top conventional stirred tank reactors (CSTRs), and an optional attrition scrubber (212), such as the one shown in FIG. 2, may comprise one or more enclosed stirred media reactors configured to be pressurized, receive oxygen, and/or contain grinding media, without limitation. In some embodiments, the optional attrition scrubber (212), such as the one shown in FIG. 2, may comprise one or more enclosed high shear stirred reactors configured to be pressurized, receive oxygen, and/or impart shear between particles of a concentrate to be leached using one or more high shear impellers, without limitation. In some embodiments, the one or more high shear impellers may be selected from the group consisting of: a Cowles disperser blade, a sawblade mixing impeller, a dispersion blade, a saw tooth dispersion blade, an angled tooth blade, an ultra-shear dispersion blade, a high flow dispersion blade, and a combination thereof, without limitation.
The processing and purification of metal sulfide-containing ores involves various unit operations, including, without limitations, crushing, grinding, and froth flotation. In the flotation process, surface-active reagents are generally used to selectively alter the wetting characteristics of sulfide mineral surfaces to promote their separation from gangue minerals. The surfactant-modified particles are separated and recovered by virtue of their selective partitioning from the mineral slurry to froth. When the mineral-containing pulp within a flotation cell is aerated, the surface-modified particles have a tendency to attach to the air bubbles, and rise by buoyancy to produce a mineralized froth which is concentrated atop the surface of the agitated, mineral pulp. Various types of froth flotation reagents are commonly used in mineral separations, including collectors, frothers, activators, and depressants.
The appearance of a stable froth is generally the end result of interfacial activity, and involves the action of surface-active species such as surfactants (i.e., amphiphilic molecules) and additionally, or alternatively, fine particles whose surfaces are amphiphilic. Conditions or phenomena which favor the adsorption of amphiphilic species at the liquid/gas interface will generally promote foam stability and frothing. Consequently, electrolyte solutions composed of ions with strong water-structure influence (i.e., positive hydration) such as SO42− would likely promote frothing, while ions which weakly influence water structure (i.e., negative hydration) such as HSO4− and SO3−, would likely be less likely to promote frothing. Additionally, high pressures suppress frothing, while atmospheric pressures or below atmospheric pressures favor its formation.
While the generation of a stable froth is used to an advantage in the selective separation and recovery of mineral particles from gangue during froth flotation processes, the appearance of a stable froth in atmospheric leach processes remains problematic. Prior art systems and methods have been proposed to deal with this problem, yet they have produced unintended problems of their own. Accordingly, new improved systems and methods are needed to overcome these problems.
In the hydrometallurgical processing of copper sulfide concentrates, a copper concentrate is typically dispersed in an acidic ferric sulfate leach liquor to bring about dissolution of copper contained in the mineral particles. The leach process produces a pregnant leach solution (PLS) which is then typically treated by a solvent extraction (SX) process to separate and recover the dissolved copper therein. The SX process is followed by electrowinning, in order to produce high-purity copper cathodes.
In some prior art leach processes (U.S. Pat. No. 5,993,635 for example), the flotation concentrate is initially subjected to ultra-fine grinding, followed directly by oxidative leaching under atmospheric conditions. In these methods, the copper is dissolved from the copper-bearing minerals at temperatures below the boiling point of water. Although there may be localized, transient heating to temperatures of 100° C. or slightly higher (due to exothermic chemical reactions), the pulp temperature is inherently limited due to the fact that the system is at atmospheric pressure. Moreover, large amounts of energy must be consumed during pre-leach ultra-fine grinding, in order to reduce particle size distributions within the flotation concentrate to a P80 of less than 20 microns, down to 5 microns.
An oxidizing agent, such as ferric ion, is commonly used to facilitate the copper dissolution reaction. During the course of this chemical reaction, the oxidizing agent (i.e., ferric ion) is reduced from the ferric oxidation state to the ferrous oxidation state. To continue the process until the majority of the copper is recovered from the mineral particles, oxygen or air is sparged into a stirred reactor to continuously oxidize the product ferrous ion back to the +3 ferric oxidation state. In the case of chalcopyrite dissolution, ferric ions are believed to enable the leaching of copper via the following reaction:CuFeS2+4Fe3+═Cu2++5Fe2++2So 
Simultaneous regeneration of the ferric oxidant and maintenance of electroneutrality is believed to proceed via the following reaction:4Fe2++O2+4H+=4Fe3++2H2O
Consequently, acid is consumed during the leaching of chalcopyrite. Similar reactions in which ferric ion acts as an oxidant are known for the leaching of other metal sulfides, including copper, zinc, iron, manganese, nickel, cobalt, etc.
During the course of the atmospheric leach process, crystalline and/or solid phase elemental sulfur (So) is produced as a reaction product by virtue of the temperatures and oxygen pressures employed. Because the temperatures involved are below the melting temperature of elemental sulfur, the sulfur appears as a crystalline and/or solid phase on the surface of the copper-bearing mineral particles. During the initial stages of the leach process, surfaces of the copper-bearing mineral particles become amphiphilic due to the appearance of the hydrophobic sulfur product. As the leach process progresses, the continued accumulation of elemental sulfur causes surfaces of the copper-bearing particles to become hydrophobic. During the early stages of the leach process, the combination of ultra-fine particle sizes, high surface areas, and the amphiphilic nature of the particle surfaces within the concentrate leads to the formation of a stable, highly mineralized froth. As a result, mineral particles trapped in the froth are significantly less likely to completely leach. During the later stages of the leach process, the accumulated elemental sulfur can also act as a physical barrier (i.e., the mineral particles passivate), thereby inhibiting continued copper dissolution from the mineral particles.
In prior art methods, the presence of flotation reagents has contributed to the problem of excessive frothing during atmospheric leaching processes of metal sulfides. This phenomenon results in metal-containing particles (for instance, copper-containing particles) becoming segregated from the leach liquor and becoming concentrated within the froth layer. This physical segregation can lead to the removal of the particles from the leach solution thereby slowing or inhibiting the copper dissolution process. In extreme cases, especially where the dissolution rate requires rapid oxygenation, frothing can be so vigorous that it becomes difficult to retain the particles within a stirred leach reactor. This leads to reduced actual residence times experienced by mineral particles residing within the leach reactor, and ultimately negatively impacts leach rates and metal (e.g., copper) recovery.
A prior art method to deal with excessive frothing includes the use of a draft tube to encourage remixing of the froth within the leach liquor. Other prior art methods focus on the use of a wetting agent within the initial stages of an atmospheric leach process, for example, Lignosol to inhibit froth stability (as described in U.S. Pat. No. 5,993,635), or calcium lignosulfonate (as described in WO 97/127070). In particular, prior art methods that teach the use of adding an attriting agent (i.e., silica sand), to enhance metal sulfide dissolution within an atmospheric leach reactor, have also necessarily required the addition of an organic defoaming agent in order to control frothing. From these prior art teachings, particulate SiO2, such as quartz and sand, are not effective defoaming agents.
Other prior art methods have similarly used wetting agents (e.g., ammonium lignin sulfonate) to mitigate the effects of sulfur passivation during elevated temperature autoclave leaching of metal sulfides where the elemental sulfur is present in a “liquid” state, rather than in a solid state (see, for example, U.S. Pat. No. 4,192,851). From prior art teachings, it can be reasoned that wetting agents such as lignin sulfonates do not effectively mitigate the effects of sulfur passivation during atmospheric leaching of metal sulfides.
Surfactants that have been found to be useful in dispersing “liquid” elemental sulfur, include, but are not limited to, lignin sulfonates, lignins, tannin compounds such as quebracho, and alkylaryl sulfonates (U.S. Pat. No. 3,867,268). In addition to surfactants, still other prior art methods which aim to promote the dispersion of liquid elemental sulfur, include ground sand, and mineral processing tailings (e.g., U.S. Pat. No. 6,497,745 and U.S. Pat. No. 7,041,152). None of these prior teachings suggest apparatus or methods for defoaming or mitigating frothing in the presence of a crystalline and/or “solid” phase sulfur product (e.g., elemental sulfur product).
A problem with prior art methods, which entail the use of surfactants to solve the problem of sulfur passivation, is the difficulty caused when these surfactants migrate to downstream unit operations, such as solvent extraction (SX) processes. For example, the contamination of an SX circuit by these surfactants can lead to the formation of oil/water emulsions that are difficult to separate, or they may lead to the formation of interfacial cruds that inhibit the interfacial mass transfer of copper. Surfactants, by their ability to adsorb at interfaces, can also interfere with the very copper dissolution reactions they are employed to aid.